1. Field of the Invention
This invention relates to a shift indicator system for a vehicle, and more particularly to a shift indicator system for a vehicle for indicating to the driver the timing to shift so that an optimal fuel-economy can be obtained while maintaining a desired running performance.
2. Description of the Prior Art
In the case of a vehicle provided with a manual transmission, the driver shifts at timings determined by his own judgment so that a desired running condition is obtained. However, if the shifting timing is not appropriate, e.g., if a lower gear speed is kept in an operating range of the engine in which a sufficient driving force can be obtained even in the next higher gear speed, fuel-economy is reduced. Thus, there have been proposed shift indicators for directing the driver to shift when it is preferred in view of the fuel-economy. See Japanese Unexamined Patent Publication Nos. 59(1984)-26332, 59(1984)-29519 and 59(1984)-29520. In the shift indicators, there is defined an engine operational range in which the engine speed is higher than a predetermined value and the throttle opening or the engine load is between predetermined upper and lower limits and in which the transmission gear should be shifted up in order to improve fuel-economy (such an operational range will be referred to as an "upshift range", hereinbelow), and when the actual engine operating condition enters the upshift range, a lamp and/or a buzzer are energized to direct the driver to upshift. The upshift range is determined in the following manner, for instance. Now, it is assumed that the engine operating condition is as represented by point A (engine speed=N, engine load=P) in the operating range diagram of FIG. 6 and that the engine operating condition is changed as represented by point B (engine speed=N', engine load=P') by shifting up the transmission gear and pushing down the accelerator pedal to maintain the vehicle speed. At this time, there must be obtained an engine output power sufficient to maintain the vehicle speed against the reduction in the engine speed and against the increase in the engine load. That is, the engine speed N' after the upshift must be higher than a predetermined value and the engine load P' after the upshift must be lighter than a predetermined value. Thus the lower limit of the engine and the upper limit of the engine load for the upshift range are determined. The lower limit of the engine load is generally set to be equal to the upper limit of the engine load defining a deceleration range so that indication of upshifting is not made during deceleration under low load. By shifting up the transmission gear when the engine operating condition is in the upshift range, fuel-economy can be improved by virtue of reduction of the sliding resistance in various parts of the engine due to reduction of the engine speed, reduction of the pumping loss due to increased throttle opening, and the like.
The indication of upshifting must be provided in such a manner that when the driver upshifts following the indication, the operating condition desired by the driver is ensured. However, in the conventional shift indicator, there has been a problem that when the engine output power properties are changed due to some reasons, the desired operating condition cannot be obtained after upshifting.
For example, when the transmission gear is shifted up at an engine speed near the lower limit of the engine speed in the upshift range or at engine load near the upper limit of the engine load in the upshift range before the engine is sufficiently warmed up, the vehicle speed cannot be maintained in response to depression of the accelerator pedal since the engine output power is generally lowered when the engine warm-up is insufficient and accordingly the required engine output cannot be obtained at the engine speed after the upshift. Similarly, when the transmission gear is shifted up at an engine load near the upper limit of the engine load in the upshift range before the engine is sufficiently warmed up, the vehicle speed cannot be maintained in response to depression of the accelerator pedal since the engine output required to match the increased engine load after the upshift cannot be obtained. In these cases, the driver must downshift soon. This will adversely affect the reliability of the indication.
Similar problems can arise when traveling at a high altitude because of reduction in the engine output due to low air density.